1. Field of the Invention
The present invention relates to video signal receivers. More specifically, embodiments of the present invention relate to methods, apparatuses, and systems for amplifying a video signal which has been transmitted over an unshielded twisted pair.
2. Description of the Prior Art
The use of surveillance cameras and related systems is becoming increasingly more prevalent. Not only are surveillance cameras used in such traditional institutions as banks and casinos, but their use has spread to retail and business establishments, indoor and outdoor entry points, airport security checkpoints, government buildings, parking lots, and the like. In most installations, it is desirable to receive images from one or more surveillance cameras in a centralized monitoring location. Depending on the installation, this centralized monitoring location may be several hundred to several thousand meters from any given individual surveillance camera. Although coaxial cable can be used as a transmission line in smaller installations, it is cost prohibitive in the majority of large installations. In these installations, the transmission line between the surveillance camera and the centralized monitoring location typically is an unshielded twisted pair (UTP). The cost of a UTP transmission line is significantly less than the cost of coaxial cable, which is due in part to the pervasive use of UTP in telephonic and computer networking systems (e.g., CAT 5/5e cables include a number of unshielded twisted pairs). In addition, UTP also has good common mode noise rejection capabilities. However, and as is common with most other types of transmission lines, a signal becomes increasingly attenuated with increasing transmission line lengths. Such attenuation must be considered in designing UTP receivers.
For long UTP transmission lines, conventional receivers employ equalizers and/or amplifiers to compensate for losses. Referring to FIG. 1, a UTP transmission line 5 may be connected on one end to a camera (not shown). At the other end, the UTP transmission line 5 can be received by a UTP receiver. The UTP receiver can include an electrostatic discharge suppression (ESD) circuit 10, and adaptive equalizer 80, and a video driver 90. The ESD circuit 10 serves to dissipate static electricity at the point the UTP transmission line is coupled to the receiver so as to prevent overvoltage and/or overcurrent damage to the receiver. Additionally, the ESD circuit 10 can include a termination circuit for load line matching. The output 95 of the video driver 90 can be coupled to a digital or analog monitor or recording device such as a video cassette recorder, digital video disk recorder, or digital video recorder.
An exemplary graph for a black-and-white composite video signal includes both high and low frequency components, as shown in FIG. 2A. A color composite video signal, as shown in the exemplary graph of FIG. 2B, also includes a color burst portion which can be seen as the high frequency portion of the signal on the left hand side of the graph. The color burst has a location within the ideal composite video waveform defined as 0.6 uS after the sync tip portion. It is further defined as a 3.58 MHz sinusoidal signal for 2.5 uS. As shown in the examples of FIGS. 2A and 2B, the portion immediately following the sync tip (the “back porch”) has a DC offset of about 300 mV (or 40 IRE) above the sync tip. The color burst is located in a section of the back porch. Ideally, the peak-to-peak amplitude of the color burst would be the same as the sync portion, or about 300 mV.
As shown in FIG. 3, attenuation in a typical UTP transmission line (and as shown, specifically Cat 5, 25-pair cable) increases logarithmically as a function of both frequency and distance. It is seen that for a given frequency, for example 10 MHz, the attenuation at 50 meters is about 2 dB whereas the attenuation at 300 meters is about 20 dB. It can also be seen that for a given transmission line length, for example 300 meters, the attenuation at 10 MHz is about 20 dB whereas the attenuation at 100 MHz is about 65 dB.
FIG. 4A is an exemplary representation of a composite video signal sweep (from zero to 6 MHz) that has been transmitted on a 300 meter UTP transmission line. It can be seen that in this example the higher frequency components of the signal (which are seen later in the time domain) are more attenuated than the lower frequency components (which are seen earlier in the time domain). FIGS. 4B, 4C, and 4D represent the same exemplary composite video signal transmitted on 600, 900, and 1200 meter UTP transmission lines, respectively. After 1200 meters, it can be seen in the example that both high and low frequency components of the composite video signal have been significantly attenuated.
Some conventional methods of compensating for UTP transmission line loss associated with the length of the transmission line include amplifying the signal received from the UTP transmission line by a gain approximately equal to the attenuation. For example, if the transmission line loss is 48 dB, conventional receiving methods would include amplifying the received signal with a gain of about 48 dB. While the line loss over a particular transmission medium can be approximated (for example, UTP line loss may be approximated as 12 dB per 300 meters at 4 MHz), it should be remembered that the quality of the transmission medium, the existence and quality of cabling connections, the proximity to sources of electromagnetic interference, and other factors may cause the actual line loss to be higher or lower than the approximated value. As such, conventional methods in which the compensating gain directly correlates only to the length of the transmission line may be inferior to other conventional methods which calculate the precise compensation by measuring the actual attenuation of the signal.
Some conventional adaptive equalizers, such as the MAXIM® MAX7474 (Trademark owned by Maxim Integrated Products, Inc.), are capable of compensating for transmission line losses of color composite signals by monitoring the sync pulse amplitude (for low frequency losses) and the color burst amplitude (for high frequency losses). However, conventional adaptive equalizers have a finite limit to the amount of compensation which can be provided. For example, the MAXIM® MAX7474 can compensate for losses on transmission line lengths of up to 600 meters by providing a high frequency gain of up to 12 dB and a low frequency gain of up to 3.6 dB. It has not been believed possible for single chip conventional adaptive equalizers to compensate for losses at sufficiently long transmission line lengths due to the number of internal digital operations which increases the overall signal to noise ratio. Thus, conventional adaptive equalizers are not suitable for use in UTP receivers where the distance between the UTP receiver and the camera (e.g., the transmission line length) is sufficiently long, for example, greater than 600 meters.
Other conventional technology includes the use of one or more amplifiers to provide a gain to the attenuated video signal prior to being received by the video drivers and/or conventional adaptive equalizers. In one example, the feedback characteristics of one or more amplifiers may be adjusted until the desired gain is achieved. For example, the feedback of any given amplifier stage may be configured so as to produce a fixed gain. When there are minimal transmission line losses (for example, when the line is less than 300 meters long), the feedback of each of the amplifier stages may be changed such that they each produce a unitary gain. However, such conventional technology unnecessarily wastes power and contributes unnecessary signal noise when any given amplifier stage is enabled but not producing a gain.
It is therefore desirable to provide methods, apparatuses, and systems which can be implemented in UTP receivers that are capable of providing substantial gain to compensate for transmission line losses resulting from long UTP line lengths while at the same time not wasting power or contributing noise when the UTP line has a short length.